Opacity information is lost when reading a P mode image with transparency information

[alexwlchan]

Describe the bug

If you have a PNG image with mode P and a custom transparency value, opacity information is lost when you convert to HEIC.

Steps/Code to Reproduce

from PIL import Image, ImageDraw
from pillow_heif import register_heif_opener

register_heif_opener()

# Draw a basic checkerboard image
im = Image.new("P", size=(100, 100))

draw = ImageDraw.Draw(im)
draw.rectangle(xy=[(0, 0),   (50, 50)],   fill=255)
draw.rectangle(xy=[(50, 50), (100, 100)], fill=255)

# Save the image as a PNG, marking the 0'th colour in the palette as transparent
im.save("checkerboard.png", transparency=0)

# Open the image, then save it as an AVIF
opened_im = Image.open("checkerboard.png")
opened_im.save("checkerboard.heic", transparency=0)

Expected Results

An image with a simple 2×2 checkerboard pattern, with transparent squares in the bottom left and top right.

Actual Results

The HEIC is rendered as a solid black image, losing the transparency from the PNG.

Versions

3.11.8 (v3.11.8:db85d51d3e, Feb  6 2024, 18:02:37) [Clang 13.0.0 (clang-1300.0.29.30)]
macOS-13.6.6-x86_64-i386-64bit
0.16.0
{'libheif': '1.17.6', 'HEIF': 'x265 HEVC encoder (3.4+31-6722fce1f)', 'AVIF': 'AOMedia Project AV1 Encoder 3.8.1', 'encoders': {'x265': 'x265 HEVC encoder (3.4+31-6722fce1f)', 'aom': 'AOMedia Project AV1 Encoder 3.8.1', 'mask': 'mask'}, 'decoders': {'libde265': 'libde265 HEVC decoder, version 1.0.15', 'aom': 'AOMedia Project AV1 Decoder 3.8.1'}}

[alexwlchan]

Note: this may have a similar underlying cause/fix to fdintino/pillow-avif-plugin#48, which is where I originally spotted this issue.

[bigcat88]

Corrected the behaviour when the transparency is 0

Thank you for the bug report.